1. Field of the invention
The present invention relates to magnetic recording media such as magnetic tapes, magnetic discs, and the like, which are obtained by applying a magnetic coating composition to a non-magnetic support.
2. Description of the prior art
In general, magnetic recording media for general purpose such as magnetic tapes and magnetic discs are produced by applying a magnetic coating composition to a polyethylene terephthalate film used as a non-magnetic support. The magnetic coating composition is usually prepared by dispersing needle-like magnetic particles having a long axis with a length of 1 .mu.m or less in a solution of resinous binder together with appropriate additives such as dispersing agents, lubricating agents, and antistatic agents.
The resinous binders used in the magnetic recording media are required, not only to yield excellent dispersibility, filling efficiency, and orientation of magnetic particles, but also to provide the magnetic coating layer with excellent durability, abrasion resistance, heat resistance, smoothness, and adhesion to the non-magnetic support. Thus, the resinous binders play a very important role. Examples of conventional resinous binders which have been hitherto used include vinyl chloride/vinyl acetate copolymers, vinylchloride/vinyl acetate/vinyl alcohol copolymers, vinyl chloride/vinylidene chloride copolymers, polyurethane resins, polyester resins, acrylonitrile/butadiene copolymers, nitrocellulose, cellulose acetate butyrate, epoxy resins, and acrylic resins.
Of these resins, polyurethane resins are excellent only in properties such as toughness and abrasion resistance due to intermolecular hydrogen bonds through a urethane linkage, compared to other resins. However, conventional polyurethane resins of adipate or caprolactone type alone are inferior in properties such as blocking resistance, heat resistance, and running stability. For these reasons, a mixed system of polyurethane resins with nitrocellulose or vinyl chloride/vinyl acetate copolymers is particularly used. Furthermore, in order to improve durability, heat resistance, adhesive properties, and the like, a curing agent is used in combination therewith. In general, polyisocyanate compounds are used as a curing agent. For the purpose of accelerating a reaction with a curing agent, aging is performed at a temperature of 40.degree. C. or higher over a long period of time, after the application and drying of a magnetic coating composition.
Conventional resinous binders are still insufficient for properties such as durability and abrasion resistance, particularly in the practical use such as video tapes, magnetic tapes for electronic computers, and floppy discs, all of which are required to have high performance and high reliability.
In addition, demand for high density and high quality magnetic recording media is increasing. Therefore, the smoothening of the magnetic coating layer is desired. In actuality, the surface smoothness of a magnetic coating layer for high density tapes and high quality tapes is being remarkably improved. As the required smoothness increases, the running durability of magnetic tapes prepared with conventional resinous binders has become insufficient, and therefore, it is desired that resinous binders with a higher durability will be developed.
In response to this requirement, an increased reactivity with a polyisocyanate compound used as a curing agent is required for resinous binders. For polyurethane resins, it has been proposed to introduce tri- or multi-functional components, such as trimethylolpropane, glycerine, diethanolamine, and pentaerythritol, into the resin. This method involves a serious drawback in that the dispersibility of magnetic particles decreases as the durability is improved; alternatively, in cases where the satisfactory durability is maintained, there is a danger that polyurethane resins might be gelled during the preparation thereof.
In recent years, fine magnetic particles made of materials such as metals and barium ferrite have been employed because of their high recording density and the high quality required for magnetic recording media. However, the performance that satisfies both durability and dispersibility is still required for resinous binders.
A method for improving the dispersibility of magnetic particles is to incorporate metal sulfonate groups or metal salt groups of an acidic phosphorus compound as disclosed in Japanese Patent Publication Nos. 57-3134 and 58-41564 and Japanese Laid-Open Patent Publication No. 61-48122. However, the introduction of tri- or multi-functional components into the resin containing metal sulfonate groups or metal salt groups of an acidic phosphorus compound for the purpose of improving the durability results in a lowering dispersibility.